buffer, to formulation solution containing stabilizer and adjuvants. All these matrices

could have a strong effect on cell-based or biochemical assays and will complexify the

extraction of meaningful data from the assays. A strong matrix effect to highlight,

especially for viral particle counting tools, is the co-secretion of extracellular vesicles

(EVs) by cultivated and infected cells while they are undergoing viral replication. Such

EVs, could be part of the host-cell response to viral infection. The main drawback of such

nanoparticle contamination is due to their physical characteristics that are very close to

the viral particles themselves. EVs are spherical particles with mean diameters ranging

from 50 nm for exosomes up to 1 µm for micro-vesicles (see Figure 8.2).

Similarly, cells following infection undergo lysis, most commonly releasing their

intracellular content in the culture supernatant. In such a case, non-assembled free

viral antigens, and viral DNA, but also host-cell DNA and host-cell proteins are

released. The viral replication process being in principle not fully efficient, it is very

common that culture broth will contain both complete viruses that are infectious and

efficient to replicate but also defective viral particles (DIPs), which will be part of a

global viral particle population. For example, the number of infectious particles

compared to defective particles released by cells could be very low. A ratio between

the infectious viral particles and the total viral particles might be as low as 1% for

an influenza viral strain (see Figure 8.4). This should also be considered as a matrix

effect for the infectious assays for example (see the section of infectious particles

assays). The ratio between the total viral particles and the infectious particles is

strongly dependent on the virus type, the virus strain, the process, or the cell pro-

duction platform used [2]. The notion of the ratio between the total viral particle and

the infectious ones is very important for monitoring and qualifying viral production

TABLE 8.1

Description of characteristics targeted by analytical tools for viral product

description and processes monitoring

Product Characteristics

Products Application

WHOLE ACTIVE VIRUS

infectious particles

complete replicative viral particles

viral genome

bioactive viral proteins

viral vectors

Attenuated viral vaccines

WHOLE INACTIVATED

VIRUS (heat or chemically

inactivated)

non-infectious particles

uncomplete viral particles

viral genome

bioactive viral proteins

inactivated vaccines

VIRUS-LIKE

PARTICLES (VLP)

non-infectious particles

Uncomplete viral particles

bioactive viral proteins

no viral genome

vaccines

VIRUS SUB-UNITS (purified

antigens or split viral

particles)

non-infectious particles

bioactive viral proteins

no viral genome

vaccines

204

Bioprocessing of Viral Vaccines